Heat-sensitive recording paper is that which forms images by utilizing a physical or chemical change of materials due to heat energy. A large number of processes involving both types of changes have been disclosed. For example, U.S. Pat. No. 3,131,080 discloses heat-sensitive recording paper which forms images by utilizing a physical change of material, and U.S. Pat. Nos. 2,663,654, 2,663,655 and 2,967,785 disclose such the paper which forms images by utilizing a chemical change of materials.
Recently, heat-sensitive recording paper has been used as recording paper for the output of facsimiles or of computers, because the paper has characteristics that the images are formed by primary coloring and a development step is not required. Such a heat-sensitive recording paper is called dye type. An example of this paper has been disclosed in Japanese Patent Publication Nos. 4160/68 and 14039/70 and Japanese Patent Application (OPI) No. 27253/80 (corresponding to U.S. Pat. No. 4,283,458) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application", and hereinafter it refers the same).
When using heat-sensitive recording paper for recording, it is advantageous because the recording apparatus can be made light weight and small. Accordingly, it has been used frequently in recent years. On the other hand, heat-sensitive recording paper is not desirable because recorded images fade under the influence of external conditions such as humidity or heat. In some instances, the fading of the recording images is fatal fault to the practical application of the recording paper. Accordingly, there have been various proposals to improve this fault.
Japanese Patent Publication No. 43386/76 (corresponding to U.S. Pat. No. 3,937,864) discloses the addition of phenol derivatives such as 4,4'-thio-bis(6-tertbutyl-3-methyl-phenol), Japanese Patent Application (OPI) No. 17347/78 discloses the addition of water-insoluble modified phenol resin such as rosin modified phenol resin. Japanese Patent Application (OPI) No. 72996/81 discloses the addition of terephthalic acid esters such as diethyl terephthalate. However, all of these methods create undesirable coloring, so-called "fogging". The fogging is caused during production or preservation of the recording paper, though these methods do have an effect on preventing fading. The fogging is substantially increased under the influence of humidity and heat. Accordingly, the commercial value of such paper deteriorates remarkably under such conditions.